Lori McIlwain, 39 of Cary, N.C., lives in constant fear that her 12-year-old son, Connor, who is autistic, will bolt from home or school if he is ever left unsupervised.

"You live in constant prevention mode," said McIlwain. "You're always on high alert."

Four years ago, Connor wandered away from a school playground and headed right toward a busy highway.

Nearly an hour later, a stranger led Connor to a school and police were called. Connor was unharmed, but for many children with autism who bolt, the outcome can be worse, even fatal.

Bolting from home is a familiar phenomenon for many families who have children with autism, but a new study now suggests these episodes happen more frequently than previously thought.

Nearly half the children who have an autism spectrum disorder (ASD) run away from home, and more than half of those who do go missing long enough to cause concern, according to a new study published Monday in the journal Pediatrics.

Families of more than 1,200 children with autism and more than 1,000 siblings without autism were surveyed online about their children's wandering habits.

Nearly half the families reported that their autistic child had attempted to escape or bolt from the home at least once after age 4, compared with only 13 percent of siblings without autism. More than a quarter of the children with autism who left their home were in danger of drowning and 65 percent were in danger of being injured by oncoming traffic, according to the study.

Anecdotal evidence suggests these episodes are all too common. On Oct. 3, a 12-year-old boy who wandered away from home in Houston died after being struck by a car while trying to cross the freeway.

"We tend to hear about the most traumatic stories on the news," said Dr. Paul Law, director of medical informatics at the Kennedy Krieger Institute in Baltimore, and study researcher. "It's just the tip of the iceberg of what parents are experiencing with this issue."

The more severe symptoms of autism, the more likely the child was to bolt, the study found.

Because the survey was administered through the Interactive Autism Network, a volunteer-based online community through Kennedy Krieger, the study may not provide a clear estimate on the wider number of autistic children who bolt and are at high risk of injury, according to the researchers.

"An unanswered question is whether the risk for elopement is higher in these children because of cognitive issues, their ASD or both," said Dr. Max Wiznitzer, a neurologist at Rainbow Babies & Children's Hospital in Cleveland.

Unlike wandering, which means to walk around aimlessly, in most cases individuals have a clear goal in mind.

For Connor, his was a fascination with highway exit signs. Connor was looking for the right number combination of an exit, McIlwain said.

"That was the turning point for us," said McIlwain, who decided to find ways to keep Connor from leaving on impulse.

Children who bolt from home are often anxious, or they enjoy running and exploring, the study researchers wrote.

"Young children in general may have a poor sense of danger, and this is particularly true for young children with autism," said Lori Warner, director of the HOPE Center at Beaumont Children's Hospital in Royal Oak, Mich., who was not involved with the study.

"Many are simply going to whatever interests them or attempting to escape unpleasant circumstances, without understanding that they could be harmed or that they should stay with their families [or] teachers," she said.

Wandering added to the parents' already high stress levels and many parents felt little support to help them manage their children's impulsive behavior, the study found.

"Taking vacations or even simple outings to restaurants or stores are fraught with worry and stress for many of our families, and as a consequence some become socially isolated, further increasing their stress and decreasing their social support," said Warner.

Currently there are no national standards for responding to missing children with autism. The Amber Alert system, which focuses on child abduction, does not cover children with autism who wander.

"Once a child goes missing, there needs to be a way to initiate a search," said Law. "Each minute that goes by without that child being recovered, the chances of a serious outcome goes up tremendously."

McIlwain uses stories to teach Connor about staying safe. They have put stop sign photos on their door as a visual cue, and installed alarms on doors and window.

Many parents like McIlwain have also found success in GPS-like tracking devices worn on the wrist or ankle, originally used as a way to locate dementia patients who wander off. When activated by the caregiver, the device generates a 911 call so the individual can be located quickly.

"Still, no measures are 100 percent effective, and parents of children with ASD must be extremely vigilant, which is draining," said Warner.

Many medical centers offer training for parents and their autistic children on preventing elopement. Children are taught signals to stop, go and stay, while parents are taught what to do if a child wanders.

Even parents who are not worried their child will take off should still learn ways to prevent and prepare, many experts said.

"Future research should focus on 'best practices' to prevent elopement, and the public also needs to become aware that children with ASD do elope, and it is not due to bad parenting," said Pauline Filipek, professor of pediatrics in the division of child and adolescent neurology at the University of Texas Health Science Center in Houston.